There is known a system which can use alcohol such as ethanol or methanol, or mixture fuel formed by mixing alcohol and gasoline as fuel in an internal combustion engine mounted in a vehicle. Such a system detects alcohol concentration of the fuel supplied to the internal combustion engine by an alcohol concentration sensor and corrects ignition timing or a fuel injection quantity in accordance with an alcohol concentration detected by the alcohol concentration sensor, thereby controlling the ignition timing or the fuel injection quantity to a proper value in accordance with the alcohol concentration. In a case where the alcohol concentration sensor is out of order, however, the ignition timing or the fuel injection quantity can not be controlled to the proper value in accordance with the alcohol concentration.
JP-4-12171A (U.S. Pat. No. 5,109,821) shows a system in which, at the abnormal time of the alcohol concentration sensor (blend ratio sensor), an alcohol concentration detection value (blend ratio) is corrected in accordance with knock information detected by a knock sensor. The ignition timing and a fuel injection quantity are set by using the alcohol concentration detection value after the correction, thus preventing occurrence of knocking.
JP-7-6430B (U.S. Pat. No. 4,957,087) shows a system in which, when it is determined that the alcohol concentration sensor is abnormal, the ignition timing is fixed to ignition timing for a gasoline engine (that is, ignition timing when the alcohol concentration is 0%) at the time of stopping an air-fuel ratio feedback control (at open-loop controlling).
According to JP-4-12171A, the ignition timing is set by using the alcohol concentration detection value corrected in accordance with the knock information when a detection accuracy of the alcohol concentration detection value deteriorates due to the abnormality of the alcohol concentration sensor. Therefore, occurrence of the knocking can be prevented by varying the ignition timing in accordance with a state of the knocking (presence/absence or intensity of the knocking). However, since the fuel injection quantity is corrected by using the alcohol concentration detection value corrected in accordance with the knock information, the fuel injection quantity is varied depending on the state of the knocking, possibly creating occurrence of a torque fluctuation or instability of a combustion state.
Generally, as the alcohol concentration of fuel supplied to an internal combustion engine becomes higher, a knock limit moves to a more advanced side. the present inventors have, as shown in FIGS. 6 and 7, studied an ignition timing control system in which the alcohol concentration (for example, ethanol concentration) becomes higher, the ignition timing is further advanced to increase torque. It is considered that also in this ignition timing control system, as described in JP-7-6430B, when it is determined that the alcohol concentration detection means such as the alcohol concentration sensor is at a failure, the ignition timing is retarded and fixed to the ignition timing for gasoline (to ignition timing when the alcohol concentration is 0%). With this way, even in a case where an actual alcohol concentration is relatively low, it is prevented that the ignition timing goes to an advance side over the knock limit, preventing occurrence of the knocking. However, in a case where the actual alcohol concentration is relatively high, a retard quantity of the ignition timing is excessively large, which possibly causes large reducing of the output torque.